The invention relates to a write-once read-many electrical memory element which comprises two electrode""s between which a layer of a material containing an organic conjugated compound is sandwiched. The invention also relates to the use of a device comprising two electrodes between which a layer containing a conjugated polymer or oligomer as well as a dopant is sandwiched, as a write-once read-many memory element. The invention further relates to an assembly of independently addressable memory elements. The invention also relates to a method of electrically writing information while using such a memory element. The invention finally relates to a method of electrically reading information while using such a memory element.
Write-once read-many electrical memory elements, as well as memories derived therefrom, in particular those in which the memory is an integrated circuit or forms part of an integrated circuit (IC) have been known for a long time. The technology which is customarily used to manufacture such memory elements is based on one crystalline silicon. However, there are applications for which silicon-based IC technology is found to be less suitable, for example xe2x80x9clow-endxe2x80x9d electronics. xe2x80x9cLow-endxe2x80x9d electronics is to be understood to mean in this context, electronics which, in terms of performance, does not have to meet high requirements and, consequently, is low in price. An example of such a xe2x80x9clow-endxe2x80x9d application is electronics intended for one-time use, such as an electronic bar code. An element which could suitably be used in such integrated circuits is a write-once read-many electrical memory element based on an organic semiconductive material.
Such an organic memory element is known per se from U.S. Pat. No. 4,371,883. This known memory element, referred to in this patent as xe2x80x9cmemory switchxe2x80x9d, is built up of a metallic substrate of, preferably, silver or copper, which substrate serves as a first electrode carrying a layer of a deposited stoichiometric metal complex of an organic conjugated electron acceptor, such as tetracyanoquinodimethane or tetracyanoethylene, which layer is in contact with a second electrode. If a sufficiently high electric field is applied, then the resistance of the layer changes. As this change in resistance remains intact for some time after the electric field has been removed, this known element is referred to as a xe2x80x9cmemory switchxe2x80x9d. However, this known memory element has a number of disadvantages. One of the disadvantages is that the change in resistance remains in tact for maximally a couple of days. Another disadvantage is that the choice of the electrode material is limited because at least one of the electrodes must be made from a metal which complexes with the organic compound, such as silver or copper. The other electrode material cannot be freely selected either because it determines the function of the device, i.e. a diode or a memory element. In addition, the materials of the two electrodes are different, which renders the manufacturing process laborious and expensive. Moreover, the known organic layer is polycrystalline and of low molecular weight. In general, such layers are brittle, so that they are less suitable to form memories having large surfaces on flexible substrates. Besides, in general the properties of devices employing organic polycrystalline layers are poorly reproducible.
It is an object of the invention to provide, inter alia, a write-once read-many memory element which does not have the above drawbacks. The invention specifically aims at a memory element which can suitably be used in an integrated circuit and in which the information remains intact for at least one month. The writing of the information should take place in a non-destructive manner so that damage to any other elements of the circuit of which the memory element could form part is precluded. It should be possible to choose and, if necessary, manufacture the electrode material and the organic layer independently. If necessary, it should be possible to make both electrodes from the same material. Further, the memory element should be stable under ambient conditions. The organic layer should be amorphous and it should be possible to provide said layer by means of simple, inexpensive techniques which are suitable for integration. An example of such a technique is spin coating.
These and other objects are achieved by a memory element of the type mentioned in the opening paragraph, which is characterized in accordance with the invention in that the layer comprises a dopant, the material is soluble, the compound is a polymer or an oligomer, and the electroconductivity of the layer in a written state is permanently lower than in an unwritten state.
As will be described in greater detail hereinbelow, it has been found that a difference in electroconductivity can be brought about by applying a suitable voltage between the electrodes. If the memory element has been subjected to such a voltage, then it is in the written state. If it has not been subjected to such a voltage, then it is in the unwritten state. A degree of conductivity in the written state, which is a factor of ten to one million lower than in the unwritten state can be easily achieved. Surprisingly, it has been found that this low conductivity remains substantially in tact for at least a month, even after the voltage is removed. The low conductivity cannot be undone by applying a voltage of comparable magnitude yet opposite polarity. Optical and electron-microscopic recordings show that the difference in electroconductivity is not accompanied by any difference in morphology of the layer, such as a phase separation or (re-)crystallization. Nor does the change lead to a loss of material from the layer or from parts of the layer. In this respect, the layer has not changed and the intended difference in conductivity has been achieved in a non-destructive manner, so that it is precluded that, if the memory element forms part of a larger assembly, other parts become damaged. The memory element proves to be stable under ambient conditions. Special measures for excluding moisture or oxygen proved unnecessary. The layer is amorphous both in the written and the unwritten state. It has also been found that in the voltage range from 0 V to approximately 1 V, the current voltage characteristic of the memory element is ohmic both in the written and the unwritten state, irrespective of the polarity of the applied voltage.
A number of different variants of the memory element can be manufactured in a simple manner. For example, a memory element can be produced in which the electrodes are coplanar and may be manufactured, if desired, from the same material. In this case, use is made of an electrically insulating substrate, such as quartz, glass or a non-conductive synthetic resin, on which the electrodes are provided in accordance with a pattern by means of known techniques, such as sputtering, CVD or vacuum deposition. Dependent upon the required dimensions, the patterns can be formed by means of known methods such as lithography, screen printing or other graphic techniques. The choice of the electrode material is not essential and, inter alia, gold, aluminum, platinum, copper or indium-tin oxide may be selected. Also conductive polymers, such as highly conductive polyaniline can suitably be used as the electrode material. This has the advantage that also the electrodes can be provided by means of simple methods, such as spin coating. Subsequently, the layer of the soluble material containing the conjugated polymer or oligomer and a dopant is provided thereon. This layer can be provided in a simple manner by means of known techniques, such as spin coating, dip coating or spray coating. Apart from wetting, the applicability of such techniques is substantially independent of the properties of the underlying layer. Layer thicknesses of approximately 30 nm to approximately 10 micrometer can be obtained in a reproducible manner, provided that the layer to be provided consists of soluble materials. A soluble material is to be understood to mean in this context, a material which can be processed by means of the above techniques. This condition is met by a material which is dissolved on a molecular level but, in this case, also by a suspension of a material. In general, conductive polymers are poorly processable owing to the rigid character of the conjugated xe2x80x9cbackbonexe2x80x9d. A known method of rendering a conductive polymer soluble consists in providing saturated substituents, such as alkyl or alkoxy groups. In this case, however, such an operation may adversely affect the conductivity. Another usable method consists in preparing a suspension of the conductive polymer.
It is known that the electroconductivity of conjugated polymers and oligomers can be increased to values ranging from 0.001 to at least 100 S/cm by adding a dopant. After the addition of a dopant, such a conductivity can be achieved with many conjugated polymers or corresponding oligomers, such as polythiophene, poly-paraphenylene, poly-paraphenylenevinylene or polyaniline, or with polymers or oligomers which are derived therefrom by means of substitution. The dopant should be selected in such a manner that, by means of a redox reaction, the conjugated polymer or oligomer is brought to a different oxidation state. Preferably, use is made of a dopant which is soluble and, apart from the desired redox reaction, does not bring about other reactions which lead to degradation of the material. If the polymer or oligomer must be oxidized, use can be made, for example, of dopants such as iodine, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), iron(III)chloride, iron(III)-paratoluenesulphonate, tetracyanoquinodimethane or nitrosoniumtetrafluoroborate. To reduce the polymer or oligomer use can be made of dopants such as alkali metals, alkali naphtalides or sodium dithionite. The electroconductivity of the layer increases as more dopant is used. In general, it appears that when the quantity of a dopant exceeds 100%, 100% corresponding to 1 electron per repeating unit, there hardly is a further increase of the conductivity. Specific conductive polymers, such as polyaniline, can also be doped with Bronsted acids, such as sulphuric acid or camphor sulphonic acid.
The same techniques and materials also enable a memory element to be obtained in which the electrodes are vertically arranged and separated from each other by the layer of doped, conjugated polymer or doped oligomer.
Using the same techniques and materials, and with similar effort, it is also possible to integrate a plurality of memory elements on one substrate.
A preferred embodiment of the memory element in accordance with the invention is characterized in that the polymer or oligomer comprises substituted or unsubstituted thiophene units. Such polymers and oligomers can be doped relatively easily, are relatively stable in the doped state and exhibit a high electroconductivity. Further, it is possible to substitute the thiophene ring; for example, to increase the solubility, without an essential change of the electronic structure.
By means of substituents in the 3rd position and/or 4th position, illustrative thiophene-containing compounds form substituted polythiophenes and oligothiophenes, said substituents either being selected independently from the group formed by R or together from the group formed by xe2x80x94Oxe2x80x94CHR1xe2x80x94CHR2xe2x80x94Oxe2x80x94 and xe2x80x94CHxe2x95x90CR1xe2x80x94CR2xe2x95x90CHxe2x80x94, and R, R1 and R2 being selected independently from the group consisting of hydrogen, alkyl, alkoxy and aryl, with the proviso that the number of non-hydrogen atoms contained in a substituent is smaller than 25 because the conductivity decreases due to the decreasing ratio between conjugated and unconjugated parts. It has been found that the use of substituents containing more than approximately 12 non-hydrogen atoms hardly improves the solubility.
A particular, preferred embodiment of the memory element in accordance with the invention is characterized in that the layer comprises a polymer in accordance with formula I or II, or an oligomer in accordance with formula III, 
wherein the degree of polymerization n ranges from 2 to 1000 and the degree of polymerization m ranges from 6 to 100.
These commercially available thiophene-containing compounds have the advantage that they are soluble. The conductive polymer in accordance with (III) is available as an aqueous suspension. A further advantage of these compounds is that the oxidation potential ranges between 0 V and +1.0 V, measured with respect to the standard calomel electrode. Compounds whose oxidation potential is in said range generally have a good stability relative to water and oxygen or a combination thereof.
Another preferred embodiment of the memory element in accordance with the invention is characterized in that the layer comprises a doped polyaniline. These compounds have the advantage that the ratio between the conductivity in the written state and the unwritten state is extremely large. In addition, doped polyaniline has a particularly high conductivity and high stability.
The invention also relates to an assembly of independently addressable memory elements, which assembly comprises memory elements in accordance with the invention. The above-described memory element can very advantageously be used if a plurality of such elements is combined in such a manner that they are independently addressable and manufactured in an integrated manner. The electrodes of the memory elements may be coplanar, said electrodes being interdigitated or not. In another configuration, the organic layer is situated between intersecting patterns of row electrodes and column electrodes, such that memory elements are defined at the locations of overlap.
The invention also relates to a method of electrically writing information while using a memory element in accordance with the invention. This method is characterized in accordance with the invention, in that a temporary voltage is applied between the electrodes, such that the electroconductivity of the layer is permanently reduced. Said permanent reduction of the conductivity is neither accompanied by a morphological change of the layer nor by any loss of any component from the layer. It is assumed that said reduction in conductivity is achieved by removing de-doping of the conjugated polymer or oligomer, possibly, in only a part of the layer. The applied voltage may be an alternating voltage or a direct voltage. In a typical example, a voltage of approximately 100 V is applied for 1 s. The conductivity value which is finally attained is governed, inter alia, by the polymer or oligomer, the type and quantity of the dopant, the voltage, the duration of the applied voltage and the layer thickness. It has been found that, at a sufficiently high voltage, there is a minimum duration txe2x80x2 for which it applies that for all durations t which comply with the relationship t greater than txe2x80x2, the conductivity changes hardly. It has further been found that a given change in conductivity can be achieved in less time by increasing the applied voltage.
The invention finally relates to a method of electrically reading information while using the memory element as claimed in claim 1, characterized in that reading takes place by measuring the electric resistance between the electrodes. The resistance of the layer can be determined in a simple manner by measuring the current at a given voltage or by measuring the voltage at a given current. If necessary, it is also possible to measure the impedance by using an alternating voltage or an alternating current. A suitable application of the memory element requires the voltages used during reading to be so low that the conductivity of the layer remains unchanged. A typical reading voltage is below approximately 10 V, in particular below 3 V.
In European Patent Application EP-A-0115191, a description is given of an electric circuit interrupter which is also referred to as xe2x80x9cfusexe2x80x9d by the Applicants. Said fuse comprises, as the active (xe2x80x9cactuatingxe2x80x9d) element, a strip of the conductive polymer polypyrrole in a doped form. The current interruption is effected by an increase in resistance of the layer, which occurs as a result of the high current running through the fuse. As a result of the increase in resistance, the quantity of current becomes negligibly small. This takes place without the element being physically destroyed or broken. In a preferred embodiment, the fuse is intentionally blown by adding a strongly oxidizing agent such as picrate. Polypyrrole is always manufactured electrochemically, resulting in a brittle or flexible self-supporting layer.
Another method of preparing an active layer of polypyrrole is neither described nor suggested in said Patent Application. It is inherent in electrochemical synthesis that material can be provided only at locations where an electrode is in contact with the starting materials used to prepare polypyrrole. This limits the number of possible embodiments, particularly the number of integrated embodiments. In particular, the memory elements in the embodiments described hereinbelow cannot be manufactured by means of said known method. Further, the known fuse is a discrete device and hence unsuitable for use as an integrated memory element. The use of the fuse as a memory element is neither mentioned nor suggested.